While the Earth's atmosphere receives electromagnetic radiation from the sun, it is itself releasing electromagnetic radiation mainly in the infrared region into space. This electromagnetic radiation interacts such as by absorption, radiation and scattering with gas molecules, clouds, aerosols and the like present in the atmosphere. By measuring changes in the spectral distribution of electromagnetic radiation caused by such interactions with the atmosphere (theoretical calculations of such distributions are referred to as radiative transfer calculations) using sensors on land or in airplanes or satellites, it is possible to study the air temperature, air pressure and concentrations of atmospheric trace gas constituents.
While a technique known as line-by-line calculation is used for radiative transfer calculations of the atmosphere, line-by-line calculations involve a enormous number of calculations. For this reason, the calculation time becomes extremely long, making it practically difficult to perform calculations over a wide spectral range at high speeds using personal computers or the like. Additionally, there are methods that make use of data obtained in line-by-line calculations beforehand involving table lookup routines as methods for avoiding the problem discussed above, but such methods have the drawback of requiring large data files. Additionally, other algorithms not requiring prior calculation have fared no better than about 20 times the speed of line-by-line calculations.
Non-Patent Publication 1: Akihiro Uchiyama, “Line-by-Line Computation of the Atmospheric Absorption Spectrum Using the Decomposed VOIGT Line Shape”, J. Quant. Spectrosc. Radiat. Transfer, Vol. 47, No. 6, pp. 521-532, 1992.